In video coding systems utilizing Motion Estimation (ME)/Motion Compensation (MC) with fractional pixel accuracy, interpolation is required to derive reference pixels at fractional pixel locations. For example, 8-tap and 4-tap DCT-based (Discrete Cosine Transform based) interpolation filter sets for fractional motion compensation have been adopted in H.265/HEVC video coding standard for luma and chroma components, respectively. An example of 8-tap interpolation filter for motion compensation is shown in FIG. 1. The thick-lines box 100 corresponds to a current 8×8 block. The 8-tap filtering of the 8×8 block requires neighboring samples as indicated by the patterned area to perform the 8-tap filtering. The horizontal interpolation for samples at A, B and C are indicated by thick-lined small squares. A group of fractional samples associated with each existing sample (i.e., integer-pel location) are generated when the fractional motion vector is allowed. For example, interpolated samples at ¼-pel, ½-pel and ¾-pel locations will be generated if ¼-pel accuracy is used for the motion vector. The interpolation can be implemented using two separate one-dimensional filters in the vertical direction and the horizontal direction. For ¼-pel, ½-pel and ¾-pel locations, three filters with individual filter coefficients are used. As shown in FIG. 1, the total number of samples includes the current block and neighboring samples involved in interpolation filtering is 15×15=225. While the fixed interpolation filter set can be implemented with lower computation complexity or less hardware cost, however, the coding performance is not very good.
In order to improve the coding efficiency related to interpolation filter for fractional-pixel motion compensation, a method of adaptive interpolation filter (AIF) has been disclosed during the development of the HEVC standard. A region-based AIF (RNAIF) has been disclosed by Matsu, et al., in JCTVC-B051 (Region-based adaptive interpolation filter, Joint Collaborative Team on Video Coding (JCT-VC), of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, 2nd Meeting: Geneva, CH, 21-28 Jul. 2010. Document: JCTVC-B051). According to JCTVC-B051, a picture is divided into multiple regions and filter coefficients are optimized on a region-by-region basis. The region-based interpolation filter has been found to improve performance around 5% to 6% in BD-rate.
While various AIF methods have shown noticeable performance improvement, it is desirable to further improve the coding efficiency without increasing the required bandwidth.